1. Technical Field
The present disclosure relates to printed circuit boards (PCBs) and, particularly, to a PCB which can improve signal integrity passing through the PCB.
2. Description of Related Art
In a PCB design process, high-speed serial signal lines are usually electrically connected to passive elements, such as resistors or capacitors, via pads mounted on the PCB.
Referring to FIG. 1, an ordinary PCB includes a signal plane 10 and a reference plane 12 which is an integrated plane without any gap arranged under the signal plane for providing a return path of signals. A passive element, such as a resistor R, and a signal transmission line 14 are arranged on the signal plane, and the resistor R is electrically connected to the signal transmission line 14 via a pad 18. A signal transmitted through the resistor R and the signal transmission line 14 will be returned in the reference plane 12 and under the signal transmission line 14 (see a return path 16). Because a width of the pad 18 is greater than a width of the signal transmission line 14, characteristic impedance changes sharply from the transmission line 14 to the pad 18, which may influence signal integrity.
FIG. 2 shows a graph of the characteristic impedance from the transmission line 14 to the pad 18 when the resistor R is a 0402 size specification, and Wpad=20 mils, Spad=54 mils, Wtrace=5 mils. Wherein Wpad is a width of the pad 18, Spad is a length of the pad 18, Wtrace is a width of the signal transmission line 14. Obviously, the signal integrity of the PCB 10 is poor.